In Rheumatoid Arthritis and Osteoarthritis, cartilage, tendon and bone are irreversibly destroyed, largely due to Matrix Metalloproteinases, MMP-1 and MMP-13. Expression of these enzymes in connective tissue cells is induced by the cytokine Interleukin-1( (IL-1(). Since joint destruction is irreversible, therapeutic strategies targeted to block this destruction are attractive. We have found that the RXR-specific ligand, LG268, and the PPAR( ligand, rosiglitazone, selectively inhibit IL-1(-induced production of these MMPs in SW-1353 chondrosarcoma cells, a proven model of chondrocyte biology, and prevent breakdown of collagen by these cells in vitro. When added together, inhibition of gene expression and collagen breakdown is greater than with either compound alone. Suppression of both genes involves changes in histone acetylation and SUMOylation of nuclear receptors. Unexpectedly, the PPAR antagonist, GW-9662, which prevents interaction of the receptor with classical DNA response elements, reduces MMP-13 expression, and further reduces MMP-1 and MMP-13 expression when combined with LG268. Thus, we hypothesize that these compounds act through novel and complex mechanisms, which include genetic and epigenetic components. Mouse models are used to study arthritis pathogenesis, and mouse and human MMP-13 share >80% sequence identity and similar expression patterns. However, mice have only a distant homologue of MMP-1, and the human MMP-1 promoter is unique. It contains a single nucleotide polymorphism (SNP), an ETS site, which is present in 75% of individuals and which augments transcription of this gene. We further hypothesize that the SNP is associated with increased cartilage breakdown in arthritis. We will, therefore, use model systems of primary cultures of human chondrocytes and murine collagen-induced arthritis to test the role of the SNP in matrix destruction and the therapeutic efficacy of LG268 and rosiglitazone in both models. To test our hypotheses, we will: (1 ).Continue to investigate the molecular mechanisms (genetic and epigenetic) by which RXR and PPAR( ligands selectively repress MMP-1 and MMP-13 gene expression;(2). Use the unique PPAR( ligand, GW-9662, to study the differential regulation of MMP-1 vs. MMP-13;and (3). Genotype DNA from human chondrocytes in order to correlate the SNP with levels of MMP-1 production and tissue destruction;and monitor the ability of RXR and PPAR( ligands to block connective tissue destruction in human cells in vitro and ablate disease in experimental arthritis in vivo. These studies will increase our understanding of how MMP-1 and MMP-13 contribute to the pathology of arthritis, and provide the rationale for future studies to test the efficacy of RXR and PPAR( ligands in patients.